Secondary Higher Invariants and Cyclic Cohomology for Groups of Polynomial Growth

TL;DR

This paper establishes new higher invariants for groups of polynomial growth, linking cyclic cohomology, K-theory, and index theory, and proves a delocalized higher Atiyah-Patodi-Singer index theorem for certain manifolds.

Contribution

It introduces polynomial growth representatives for delocalized cyclic cocycles and defines higher eta invariants, connecting them with K-theory and index theory for polynomial growth groups.

Findings

Existence of polynomial growth representatives for delocalized cyclic cocycles.

Definition of higher eta invariants with proven convergence.

A delocalized higher Atiyah-Patodi-Singer index theorem for specific manifolds.

Abstract

We prove that if $\Gamma$ is a group of polynomial growth then each delocalized cyclic cocycle on the group algebra has a representative of polynomial growth. For each delocalized cocyle we thus define a higher analogue of Lott's delocalized eta invariant and prove its convergence for invertible differential operators. We also use a determinant map construction of Xie and Yu to prove that if $\Gamma$ is of polynomial growth then there is a well defined pairing between delocalized cyclic cocyles and $K$-theory classes of $C^*$-algebraic secondary higher invariants. When this $K$-theory class is that of a higher rho invariant of an invertible differential operator we show this pairing is precisely the aforementioned higher analogue of Lott's delocalized eta invariant. As an application of this equivalence we provide a delocalized higher Atiyah-Patodi-Singer index theorem given $M$ is a compact spin manifold with boundary, equipped with a positive scalar metric $g$ and having fundamental group $\Gamma=\pi_1(M)$ which is finitely generated and of polynomial growth.